Laser scanners are currently being used for numerous applications, including electronic component marking, fine engraving, micro-assembly soldering and welding, scribing and various other repetitive production operations involving near infrared or far infrared lasers. Laser scanners employ optical elements, usually mirrors and lenses, to direct a laser beam at an object being scanned. The optical elements are programmable to allow the surface of the object to be scanned in two dimensions.
A typical prior art laser scanner is shown in FIG. 1. A laser source (not shown), typically a Yttrium-Aluminum-Garnet (YAG) laser or Carbon Dioxide (CO.sub.2) laser, supplies an input laser beam 10 to a refractive beam expander 11. The beam expander 11 typically uses a negative input lens 12 and a positive collimation lens 14 to magnify the input laser beam. The laser scanner includes two galvanometer-operated mirrors positioned along the optical axis, one mirror 16 deflecting the expanded beam in an X dimension and the other mirror 18 deflecting the beam in a Y dimension. A focusing lens 20 focuses the twice-deflected laser beam onto the object being scanned. The focusing lens typically is of flat field F.theta. type, having approximate proportionality between input field angle and image displacement.
Each mirror is controlled by a computer-driven servo that corrects for distortions in the system. Typically, the servo includes a microprocessor coupled to read-only memory (ROM) that stores a lookup table calibrated to provide correction values for every combination of mirror angles. The microprocessor is programmed with appropriate software that looks up the correction values for a given combination of mirror angles.
Several deficiencies exist in such prior art systems. One deficiency is that the system is overly complex, and therefore expensive, due to the large number of infrared lenses needed to implement the beam expander and the focusing lens. These infrared lenses must be anti-reflection coated and are usually changed for different laser wavelengths. A second deficiency is that the X and Y positioning is accomplished in axially spaced galvanometer-driven mirrors, causing pupil astigmatism with the imaging/focusing lens. The imaging/focusing lens and scanning mirrors have to be made larger to accommodate the axially shifting pupil. This causes optical inefficiencies and the use of more expensive infrared glass. The ill-defined optical pupil also causes optical distortions that must be removed with the ROM look up tables.
A third deficiency is that using software to obtain correction values suffers from speed limitations. The speed at which corrected mirror values can be presented to the mirrors is reduced because computer code is required to implement the software correction regardless of the, software algorithm employed. In a high performance scanning system such reduced speed provided by software correction schemes is unacceptable. Near real-time software correction is possible using plural microprocessors, but the additional microprocessor greatly increases the cost and complexity of the system.